The culture of some zooblast or animal cells, microorganisms, fungi and plant cells is anchorage dependent; the cells need a surface to grow on. One way that this is done is through the use of cylindrical roller bottles as horizontally rotated bioreactors. A liquid nutrient medium inoculated with cells fills the horizontal bottle to a shallow depth (below the bottle opening) and the bottle is rotated by two horizontal drive rollers upon which it rests. Some bottles are provided with protruding drive rings or collars at the perimeter of the top and bottom of the cylinder. Gas exchange, oxygen in and carbon dioxide out for mammalian cells, is generally accomplished at a breather cap on the bottle, which, for example, is provided with a hydrophobic porous membrane and one or more holes at the end of the cap. Alternatively, the gas exchange can be accomplished by diffusion through the walls, augmented by the presence of micropores in the walls. After a time, the cells that have grown on the inside walls are harvested by scraping them off with a blade. Increasing the surface area to volume ratio of the bottle increases the yield of cells from each bottle. In the past this has been accomplished by providing the surface of the bottle with ribs, pleats, ridges, grooves, flutes or corrugations that run either perpendicular or parallel to the axis of the bottle. Despite the introduction of ribbed bottles, smooth wall bottles are still sold. This may be because they are easier to harvest and clean, especially when using automation. Also, reusable bottles such as glass may be impractical to mold with ribs, in comparison to the disposable plastic bottles. Surface area increase is also achieved by using a bioreactor shaped as hollow cylindrical annulus, and by putting packing material or brushes inside the bottle. Agitation of the solution has been increased by the use of inserts and high rotation speeds. The bottles are usually constructed of either glass, stainless steel or a clear plastic, such as polystyrene, polyurethane, polyvinyl chloride, polycarbonate, polyolefins such as polypropylene, polyethylene terephthalate with glycol additives, ethylene glycol- 1,4, cyclohexane dimethanol terephthalate copolyester and the like. Transparent materials are preferred, as cell growth can be monitored by placing the bottle on an inverted microscope. Cell adhesion is improved by treatment with silane coupling agents, plasma etching and fibronectin treatment, for example. For repeat use, glass vessels are sometimes siliconized to prevent the surface from becoming rough.